The invention relates to a magnetic resonance imaging apparatus (MR apparatus) which is provided with an open magnet system. Magnet systems of this kind are essentially C-shaped and comprise an upper section and a lower section which both extend in the horizontal direction and are interconnected by way of a vertical column. An object to be examined (a patient) is arranged between the horizontal sections. Because the object to be examined is readily accessible from all sides, such systems are referred to as open magnet systems (as opposed to systems having a tubular examination zone); one or more further vertical columns may also be provided, for example, in order to enhance the mechanical stability of the magnet system.
Pole plates for generating a basic magnetic field (B0-field) as well as gradient magnetic fields are provided on the horizontal sections of such magnet systems. The basic magnetic field extends through the patient essentially in a direction perpendicular to the longitudinal axis of the patient (that is, generally in the vertical direction).
Flat, or at least flattish, RF conductor structures (flat resonators) in the form of RF transmitter coils or RF receiver coils are used to generate an RF field (B1-field) as well as to detect MR relaxation processes, said coils being provided on the pole plates. Furthermore, RF receiving coils can also be arranged around a region to be examined.
The MR frequency is dependent on the strength of the basic magnetic field. For the field strength of approximately 0.2 Tesla as customarily used nowadays, an MR frequency of approximately 8.2 MHz is obtained. In order to enhance the image quality, the aim is generally to increase this field strength to a range of up to approximately 1 Tesla; this value would give rise to an MR frequency of approximately 42.3 MHz. However, such a frequency would give rise to numerous problems during operation of an MR imaging apparatus of the known kind.
A first problem is due to the fact that the magnet system itself constitutes an electrical resonator, because the C shape has the effect of a folded dipole. The self-resonant frequency of a superconducting C-shaped magnet system is in the range of from 40 to 50 MHz in the case of customary dimensions of an MR apparatus which is intended for the examination of humans, so that it is of the same order of magnitude as the MR frequency at a field strength of the basic magnetic field which amounts to 1 Tesla (42.3 MHz). Consequently, electromagnetic energy can move from a corresponding RF transmitter coil into the magnet system and excite resonances therein which may give rise to strong electrical and magnetic fields on the enclosure of the magnet, and notably on the free ends of the C shape (so-called hot spots). Furthermore, the RF receiving coils could become coupled to the magnet system, so that the signal-to-noise ratio deteriorates and imaging is affected.
An additional problem consists in that, generally speaking, the RF transmitter coils are arranged directly on the pole plates where a current maximum of the dipole structure formed by the magnet system occurs, so that the RF transmitter coils are particularly sensitive to the resonant dipole structure and hence a comparatively strong coupling occurs and also strong influencing of the RF transmitter coils. Consequently, quadrature coil systems present in the magnet system may thus even be prevented from generating a circularly polarized RF field, because the coupling to the dipole field eliminates the orthogonality.
A further problem consists in that, as opposed to imaging apparatus provided with a tubular examination space, the RF conductor structures in open MR systems radiate electromagnetic energy essentially freely to the surrounding space. The space in which an MR imaging apparatus is installed, therefore, must always be shielded so as to prevent the electromagnetic radiation from interfering with the surroundings. Such a space (RF cage), however, at the same time constitutes a cavity resonator which is capable of absorbing energy. The fundamental mode of such a space, typically having the dimensions of 5 by 5 by 3 meters, amounts to approximately 42 MHz, so that space resonance can be excited and couplings between the RF structures and the space, being tuned to the (essentially the same) RF frequency, are unavoidable.
Thus, increasing the field strength of the basic magnetic field would give rise to numerous problems in the known MR imaging apparatus.
Therefore, it is an object of the invention to provide an MR imaging apparatus of the kind set forth which can operate with significantly higher field strengths (that is, field strengths of up to 1 Tesla or more) of the basic magnetic field, that is, essentially without interference and with a higher image quality.
This object is achieved by means of a magnetic resonance imaging system which is provided with an open magnet system as disclosed in claim 1 and is characterized in that coupling effects between a dipole structure formed by the magnet system and an RF conductor structure tuned to an MR frequency are eliminated at least to a high degree by shifting or detuning a self-resonant frequency of the dipole structure relative to the MR frequency.
In conformity with a further solution as disclosed in claim 2, such coupling effects are eliminated at least to a high degree by suppression of a self-resonant frequency of the dipole structure.
If necessary, the above two steps can also be combined.
The dependent claims relate to further advantageous elaborations of the invention.
The claims 3 to 6 disclose advantageous types of shifting or detuning of the self-resonant frequency of the dipole structure by changing the electrical length thereof.
The claims 7 to 10 relate to the suppression of the self-resonant frequency of the dipole structure by way of a standing wave barrier or impedance trap and its advantageous embodiments.
In order to optimize its effect in respect of avoiding couplings between the magnet system and the RF conductor structure, the impedance trap is preferably tuned to the MR frequency in conformity with claim 8.
The embodiments disclosed in the claims 9 and 10 are advantageous in particular when the geometrical length of the impedance trap deviates from the electrical length actually required.
The embodiment disclosed in claim 11 prevents emitted electromagnetic radiation from interfering with the surroundings of an RF cage and also prevents the formation of standing waves within the RF cage.